Thin film optical waveguides for coupling light between two points are generally well-known in the art. Typical optical devices which make use of light transmission in thin films include light guides, light modulators, directional couplers, optical polarizers, and other similar devices. These devices usually include one or more thin films which are typically either dielectric or semi-insulating films which are deposited on a chosen substrate and have a film thickness on the order of the wavelength of light to be transmitted therethrough. Light introduced into one end of the film will be propagated to the other end of the film if the film material has a certain minimum absorption per unit length, which in the case of a semiconductor film implies a certain minimum bandgap energy. The coupling efficiency for this light propagation is dependent upon the amount of light that is reflected by the surface barriers on opposing sides of the film. This quantity is, of course, dependent upon the difference between the indices of refraction of the film material per se and of the material adjacent thereto, whether it be air or some adjacent integral material such as a semiconductive substrate. As this difference between adjacent indices of refraction increases, the coupling efficiency for light propagated through the film increases.
Gallium arsenide (GaAs) is a suitable semiconductive material from which these thin films can be fabricated, and GaAs has a bandgap energy suitable for sustaining light propagation at a relatively high efficiency. That is, the bandgap energy of gallium arsenide is approximately 1.4 electron volts (corresponding to an absorption edge of 8900 .ANG.) which is somewhat greater than the photon energy, for example, from a helium-neon light source (1.075 eV) or from a Nd:YAG source (1.18 eV). Thus light from either of these two commonly used sources can be propagated in GaAs without excessive absorption. However, as will become apparent herein, the waveguide structures according to the present invention can also be fabricated from gallium phosphide, which has a bandgap energy of 2.24 eV. GaP allows high efficiency (low absorption) propagation of visible light with wavelengths as short as .gtoreq.5500 .ANG. (green). Additionally, aluminum arsenide (AlAs), which has a bandgap=2.4 eV and will transmit light of wavelengths .gtoreq.5200 .ANG. (blue-green), is also a suitable semiconductive material. The ternary compounds Ga.sub.(1-x) AlxAs and GaAs .sub.x P.sub.(1-x) can also be used and have absorption edge wavelengths intermediate to those for GaAs and GaP or GaAs and AlAs, depending on the molar fraction (x) of the third element.